A tunnel junction is a barrier, such as a thin insulating layer or electric potential, between two electrically conducting materials. Electrons (or quasiparticles) pass through the barrier as, according to quantum mechanics, each electron has a non-zero wave amplitude in the barrier, and hence some probability of passing through the barrier. Accordingly, tunnel junctions have been employed within a broad range of electronic and optoelectronic devices. However, its application in wide bandgap devices, such as those exploiting GaN and AlN semiconductors, has been limited to date. These materials are of interest as mixture of GaN with In (InGaN) or Al (AlGaN) with a band gap dependent on the ratio of In or Al to GaN allows the manufacture of light-emitting diodes (LEDs) and lasers with colors that can go from red to ultra-violet (UV-A/UV-B; 280 nm ≤λ≤400 nm) whilst AlN with Ga LEDs and/or lasers can span into the deep ultra-violet (UV-C; 200 nm ≤λ≤280 nm).
However, to date, it has remained challenging to form an efficient tunnel junction using either AlN or GaN -based semiconductor materials. Now referring to FIG. 1A there is depicted the schematic energy band diagram of a conventional n++-GaN/p++-GaN tunnel junction (TJ). The inefficient p-type doping in GaN and AlN, due to the low acceptor ionization efficiency, leads to a large depletion region width (W, i.e. wide tunnel barrier thickness) that inhibits efficient inter-band tunneling. In this context, the inherent spontaneous and piezoelectric polarization of wurtzite GaN-based heterostructures has been exploited to enable efficient inter-band tunneling within the prior art. Several tunnel junction designs, including GaN/AlN/GaN, GaN/InGaN/GaN, AlGaN/InGaN, AlGaN/GaN, and GaN/InGaN have been demonstrated within the prior art. Although promising results have been achieved using these tunnel junctions in planar devices, it has remained challenging to incorporate these designs into emerging high efficiency nanowire structures. Due to the inherent strain relaxation, the polarization-induced sheet charge at the heterointerface of nanowire structures is significantly reduced. Consequently, polarization engineered tunnel junctions may exhibit a higher voltage drop in nanowire-based devices. In addition, the successful incorporation of such tunnel junctions by and large depends on the crystal polarity (N-face or Ga-face). Despite the reduction in tunnel barrier width over conventional tunnel junctions, the afore-described AlN/GaN and InGaN/GaN based tunnel junctions still suffer from comparatively low inter-band tunneling conduction as well as optical absorption loss. Alternatively, within the prior art, tunnel junction devices incorporating rare earth materials such as GdN or semi-metallic MnAs and ErAs nanoparticles have been demonstrated under forward and reverse bias, wherein tunneling is enhanced by the presence of mid-gap states.
However, it would be beneficial to provide designers of nanowire based electronic and photonic devices with tunnel junctions that do not require the incorporation of rare earth nanoparticles, exhibit low inter-band tunneling conduction or exhibit optical absorption loss. It would be further beneficial to provide designers with an intrinsic molecular beam epitaxy (MBE) compatible tunnel junction exploiting a metal based layer supporting use over a wide range of GaN and/or AlN compositions. Further, it would be beneficial, within photonic devices, for the metal based layer to reflect light emitted from the nanowire active region in direct contrast to the light absorption induced by polarization engineered tunnel junctions within the prior art such that high reflectivity in the visible and UV spectral range can improve device performance.